
#include "nrutil.h"

void rk4(y,dydx,n,x,h,yout,derivs)
float dydx[],h,x,y[],yout[];
int n;
void (*derivs)();
{
	int i;
	float xh,hh,h6,*dym,*dyt,*yt;

	dym=vector(1,n);
	dyt=vector(1,n);
	yt=vector(1,n);
	hh=h*0.5;
	h6=h/6.0;
	xh=x+hh;
	for (i=1;i<=n;i++) yt[i]=y[i]+hh*dydx[i];
	(*derivs)(xh,yt,dyt);
	for (i=1;i<=n;i++) yt[i]=y[i]+hh*dyt[i];
	(*derivs)(xh,yt,dym);
	for (i=1;i<=n;i++) {
		yt[i]=y[i]+h*dym[i];
		dym[i] += dyt[i];
	}
	(*derivs)(x+h,yt,dyt);
	for (i=1;i<=n;i++)
		yout[i]=y[i]+h6*(dydx[i]+dyt[i]+2.0*dym[i]);
	free_vector(yt,1,n);
	free_vector(dyt,1,n);
	free_vector(dym,1,n);
}
